The field of the invention is nuclear magnetic resonance (NMR) techniques for producing angiograms.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor Frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the paramagnetic nuclei in the tissue attempt to align with this field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented components in the perpendicular plane (x-y plane) cancel one another. If, however, the substance, or tissue, is irradiated with a magnetic field (RF excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment M.sub.z can be rotated into the x-y plane to produce a net transverse magnetic moment M.sub.1 which is rotating in the x-y plane at the Larmor frequency.
The practical value of this gyromagnetic phenomena resides in the radio signal which is emitted after the RF excitation signal is terminated. When the excitation signal is removed, an oscillating sine wave is induced in a receiving coil by the rotating field produced by the transverse magnetic moment M.sub.1. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of M.sub.1.
The measurements described above are called "pulsed NMR measurements." They are divided into a period of excitation and a period of emission. As will be discussed in more detail below, this measurement cycle may be repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject.
Although NMR measurements are useful in many scientific and engineering fields, an important use is in the field of medicine. NMR measurements provide a contrast mechanism which is quite different from X-rays, and this enables difference between soft tissues to be observed with NMR which are completely indiscernible with X-rays. In addition, physiological differences can be observed with NMR measurements, whereas X-rays are limited particularly to anatomical studies.
For most medical applications utilizing NMR, an imaging technique must be employed to obtain information at specific locations in the subject. The foremost NMR imaging technique is referred to as "zeugmatography" and was first proposed by P. C. Lauterbur in a publication "Image Formation by Induced Local Interactions: Examples Employing Nuclear Magnetic Resonance", Nature Vol. 242, Mar. 16, 1973, pp. 190-191. Zeugmatography employs one or more additional magnetic fields which have the same direction as the polarizing field B.sub.0, but which have a nonzero gradient. By varying the strength (G) of these gradients, the net strength of the polarizing field B.sub.0 =B.sub.z +G.sub.x X+G.sub.y Y+G.sub.z Z at any location can be varied. As a result, if the frequency response of the receiver coil and circuitry is narrowed to respond to a single frequency .omega..sub.0, then gyromagnetic phenomena will be observed only at a location where the net polarizing field B.sub.0 is of the proper strength to satisfy the Larmor equation; .omega..sub.0 =.gamma.B.sub.0 : where .omega..sub.0 is the Larmor frequency at that location.
By "linking" the resulting NMR signal with the strengths of the gradients (G=G.sub.x, G.sub.y, G.sub.z) at the moment the signal is generated, the NMR signal is "tagged", or "sensitized", with position information. Such position sensitizing of the NMR signal enables an NMR image to be reconstructed from a series of measurements. Such NMR imaging methods have been classified as point methods, line methods, plane methods and three dimensional methods. These are discussed, for example, by P. Mansfield and P. G. Morris in their book NMR Imaging in Biomedicine published in 1982 by Academic Press, New York.
The NMR scanners which implement these techniques are constructed in a variety of sizes. Small, specially designed machines are employed to examine laboratory animals or to provide images of specific parts of the human body. On the other hand, "whole body" NMR scanners are sufficiently large to receive an entire human body and produce an image of any portion thereof.
An angiogram is a visualization of blood vessels. Traditionally, angiograms are produced by injecting the patient with a radiopaque substance and then taking an X-ray of the patient from the desired projection angle. The radiopaque substance flowing in the blood vessels is opaque to the X-rays, and the cardiovascular system appears brighter than the surrounding tissues in the resulting image. While high resolution angiograms may be produced with this conventional method, the patient is subjected to ionizing radiation.
Two methods have been used to produce angiograms by exploiting the NMR phenomenon. One of these is referred to as the "time of flight" or "inflow enhancement" method for contrasting flowing spins from the surrounding stationary spins, and the other is referred to as the "phase contrast" method.
Inflow enhancement occurs when unsaturated spins flow into a slice which has been excited by many radiofrequency (RF) pulses. If the time between RF pulses is much shorter than the T.sub.1 relaxation rate of the tissues, the longitudinal magnetization does not have time to recover before the next RF pulse is applied. This results in reduced transverse magnetization and reduced signal when the magnetization is again tipped into the transverse plane by the next RF excitation pulse. The in-flowing blood, on the other hand, will have seen no prior RF pulses and will therefore have a large longitudinal component of magnetization, which produces a larger transverse magnetization and a larger NMR signal. As a result, the flowing blood appears brighter in the reconstructed image.
In order to produce an angiogram, using the time-of-flight method, a series of contiguous thin slices oriented perpendicular to the direction of primary blood flow is collected. The slices can be collected sequentially in a 2D fashion or simultaneously in a 3D fashion. If 3D acquisition is used, the slice must be on the order of a few centimeters thick and the flip angle of the excitation pulses must be reduced. These measures are necessary to prevent saturation of the blood as it traverses the slice. Once the NMR data from the slices is collected, a projection is produced using a ray tracing technique. The most commonly used technique involves tracing a ray through the slice data and retaining the value of the most intense pixel encountered. The pixel associated with each ray is then mapped to its corresponding position in the projection image.
In phase contrast angiography, the mechanism for flow contrast is modulation of the phase of the transverse magnetization. The objective is to alter the phase of the NMR signal produced by the moving spins, while at the same time leaving the net phase of the NMR signal produced by the stationary spins unchanged. This result can be achieved through the use of a bipolar gradient waveform.
For a bipolar gradient waveform such as that shown in FIG. 3B, with first moment M.sub.1 =At, it is well known that the NMR signal produced by a spin moving with velocity v along the axis containing the bipolar gradient will accumulate a phase given by the following equation: EQU .phi.=.gamma.M.sub.1 v (1)
where .phi. is the phase accumulation and .gamma. is the gyromagnetic constant of the spins.
Because stationary spins have velocity equal to zero, the phase change imparted to these spins will be zero. The sign of the phase change acquired by a moving spin is dependent on the sign of the amplitude of the lobes of the bipolar gradient. Therefore, if two excitation sequences are performed, each containing a bipolar gradient of opposite sign, each will impart equal but opposite phase shifts to the moving spin. Upon subtraction of the NMR data obtained using these two sequences, signal from stationary tissues will be cancelled whereas signal from moving spins will be reinforced to enhance the image.
As with the time of flight methods, phase contrast angiograms can be obtained using 2D or 3D acquisition schemes. Because with phase contrast techniques, the contrast does not rely solely on inflow phenomenon, a much larger volume is usually imaged in a single acquisition. In order to prevent saturation of the blood, a tip angle of approximately 30.degree. is used. The smaller tip angle results in less blood signal than is obtainable with the 60.degree. flip angle used in the time of flight technique, but because phase contrast is a subtraction method, the elimination of signals from stationary tissues makes the contrast associated with this technique far superior to that associated with the time of flight techniques.
The above described sequences work well in regions void of respiratory motion and vessel motion caused by cardiac pulsatility. However, when these techniques are applied to the abdomen or extremities where respiration and/or pulsatility are present, they yield images which are far from acceptable. One problem is image artifacts caused by the motion of spins. As indicated above, movement of the spins imparts a phase change to the NMR signal and this is indistinguishable from the phase imparted to the NMR signal due to the phase encoding gradient. As a result, when the image is reconstructed the position of the spins are misplaced in the phase encoding direction to produce ghosting or blurring in the image. A common solution to this problem is to employ respiratory or cardiac gating to the data acquisition procedure so that NMR data is acquired at times during the cycle when movement of the vessels is at a minimum.
Another problem with angiograms produced by the phase contrast method is that the image contrast varies as a function of blood velocity. Thus, NMR data acquired during the diastolic portion of the cardiac cycle when blood velocity is low will lack contrast. As a result, the blood vessels may not differ at all from the stationary background tissues. On the other hand, if the first moment M.sub.1 of the bipolar gradient is increased to improve contrast at low velocities, then signal strength is lost at high blood velocities due to an excessive phase accumulation (i.e. aliasing). As a result, the angiogram will show parts of the vascular system very brightly and other parts will fade into the background. This problem is particularly acute at the extremities where blood velocity varies considerably during the cardiac cycle.